Bridge circuits with an electrical motor mounted in the crossarm are known from various power electronic circuits. A so-called full-bridge circuit normally consists of four electronic circuits in the bridge branches, of which two at a time are switched in a bridge branch between the supply voltage and the mass. These switches allow to control not only the direction of the motor current in the crossarm but also its magnitude, making it possible to operate the drive system in all four quadrants of the rotational speed-torque-family of curves.
In automobile technology such bridge circuits are used with comfort systems and serve for example as a drive system for separately controlled power windows, seat adjustors, sliding roofs or belt rollers. With an electrical power window it is necessary, for safety reasons and over a certain range of adjustment, to have an effective protection against being caught in it. The drive system is therefore designed in such a way that the sense of rotation of the motor is reversed as soon as a slowdown in rotation is detected. For a spring rate of 10 Newton per millimeter the squeezing force may not exceed 100 Newton. This function is ensured by an electronic control that monitors the engine revolutions during the adjustment. The control is often installed away from the center and integrated into the power window motor, minimizing thereby the need for wiring within the vehicle. The speed of the adjustment drive is regulated by a semiconductor device switched into the supply line to the battery, for example a timed power field effect transistor, also called chopper. So-called polarity-reversing relays are used to reverse the polarity of the motor. These are electromechanical polarity-reversing devices consisting of two electromagnetic systems, of which one operating contact each is connected to the crossarm of the bridge circuit and in which one inoperative contact each is switched into a bridge branch. One electromechanical polarity-reversing relay such as normally used for drive devices in motor vehicles, is described in EP 055 6 540 B 1.
When timing a direct-current motor mounted in the crossarm of a bridge circuit, so-called freewheeling diodes are necessary that take over the motor current during those switching phases during which the chopper severs the connection to the battery. These freewheeling diodes are connected in parallel to the bridge branches. A freewheeling diode is an additional component that causes additional costs and requires the corresponding mounting space on the controller. However, if the controller is integrated into the motor housing, the mounting space is very limited. Also, there is substantial cost pressure on all control electronics, as there is also on other components of the drive system. Efforts are always undertaken during the development phase to achieve the desired functionality with the lowest number of components and the least expensive components.